Blood meal is an organic fertilizer known for its high nitrogen content, making it a beneficial amendment for growing onions. Onions are heavy feeders, especially early in their growth cycle. The effectiveness of blood meal depends entirely on precise application timing and dosage. Correct use supports the robust vegetative growth necessary for a successful harvest, while misuse can negatively impact the final bulb quality.
The Role of Nitrogen in Onion Growth
Nitrogen is a primary nutrient that dictates the success of an onion crop, particularly during the initial vegetative stage. This element drives the development of the plant’s green tops, which are responsible for photosynthesis and energy production. The size of the mature onion bulb is directly correlated with the number of healthy leaves the plant develops before bulbing begins.
Each leaf corresponds to a potential layer in the final bulb, meaning maximizing foliage leads to a larger storage organ. Nitrogen ensures the leaves are vigorous and healthy, maximizing the plant’s ability to store energy for the later bulbing phase. The goal is to maximize leaf production before day length and temperature trigger the shift to bulb formation.
Blood Meal Application and Timing
Blood meal is a concentrated organic source of nitrogen, often having an NPK ratio of 12-0-0 or 13-0-0. Its nitrogen is released relatively quickly as microorganisms break it down, providing an immediate boost to young, establishing onion plants. This rapid availability makes it suitable for the crop’s high nitrogen demand during the early season.
The most impactful time to apply blood meal is in the early season, starting a few weeks after planting or transplanting. Side-dress the plants every three to four weeks during the vegetative growth phase until the bulbs begin to form. A general guideline is to use about 1 cup per 10 feet of row, or 1 to 2 pounds per 100 square feet, lightly working it into the top few inches of soil.
Mixing the blood meal into the soil or side-dressing established plants is preferred over broadcasting it on the surface. Watering immediately after application helps the nitrogen become available to the plant roots more quickly. Since the nitrogen typically dissipates within about six weeks, split applications are necessary to maintain a consistent supply during the crucial growth stage.
Avoiding Nitrogen Overload
While nitrogen is necessary for growth, applying too much or applying it too late can cause negative consequences for onion quality and storage. The most significant risk is the delay of bulb formation, as excessive nitrogen encourages the plant to focus energy on producing leaves instead of swelling the bulb. Continuing a high-nitrogen schedule too late can lead to thick-necked onions, which do not cure or store well.
Nitrogen overload also negatively affects the quality of the mature bulb, resulting in softer bulbs with a reduced shelf life. These softer bulbs are more susceptible to rot and sprouting in storage. Over-application of blood meal can also cause fertilizer burn, where the high concentration of salts damages the foliage and roots. To prevent these issues, all nitrogen fertilization should cease entirely once the onion plant begins the visible transition into the bulbing phase.